Spinal fusion surgery employs different devices for achieving fusion of two or more vertebrae. For example, intervertebral cages are inserted in the disc space, as spacers, to improve the anterior column stability, preserve the disc space height and to contain bone graft in dedicated chambers to enhance the fusion process. In some cases supplementary screws, in the facets, pedicles or in the vertebral body from the anterior or lateral sides, are added to increase the stability. Some intervertebral cages are made of a single segment device, whereas others have more than one cage to cover more surface. Multiple segments may be attached with hinges, pivots or turning points to be able to cover more of the surface with only one device.
Intervertebral cages should provide a wide foot print for contact surface with both vertebrae endplates (superior and inferior). The cages should be placed preferably in the periphery of the disc space and should be able to fit the disc space height. Some cages provide expansion mechanisms for adjusting the height to fit the disc space height. Height adjustment is sometimes complemented with correction of lordosis by providing some angulation according to the location. In other cages, horizontal expandable mechanisms are provided for expansion transverse to the height. However, locking the expandable mechanism for vertical (height) or horizontal expansion may present a challenge due to the physiologic forces involved.
U.S. Pat. No. 6,332,895 describes an expandable intervertebral implant that includes a pair of semi-cylindrical shells, which are distracted inside an intervertebral space that has been appropriately prepared for fusion from an anterior approach. The semi-cylindrical shells have lateral wings which sit juxtaposed to the end plates of the vertebral body to provide a large surface area for adjacent end plate support. Each wing makes up at least 20% of the width of the implant, preferably at least 25%. The shells are distracted with an expandable installation tool and the shells are held apart by ratchets or corrugations in their side walls to permit optimal tensioning of the annular support ligaments, and hence immediate stability. The installation tool is then unscrewed and disengaged, leaving the component parts as a stable assembly that can be packed with bone to promote osseous union.